ENGR00221440 MX6x-Fix race-condition in checking bus_freq variables

ENGR00221440 MX6x-Fix race-condition in checking bus_freq variables

Checking of the bus_freq variables and changing of the
bus/ddr frequency should be done under one mutex.
Else there is a race-condition that the variable changed
just after it was checked.
Also ensure that the bus freq is always increased before
the cpu freq is set to anything other than the lowest setpoint.
Else there is a possibility that the ARM is set to run from
PLL1 at higher frequency when bus/DDR are still at 24MHz.
This is dangerous since when system enters WAIT mode in
low bus freq state, PLL1 is set to bypass when ARM is being
sourced from it.

Signed-off-by: Ranjani Vaidyanathan <ra5478@freescale.com>